The present invention relates to a safety-enhanced protective circuit for a neon transformer which is used to energize a neon tube or an argon tube for illumination.
FIG. 1 shows a conventional circuit of the kind which detects a ground fault of a neon transformer. A leakage transformer (non transformer) 11 includes a primary winding 12 having one connected through a switch 13 to an input terminal 14 and the other end connected to an input terminal 15. The transformer also includes a pair of secondary windings 16, 17 having their starting ends connected together at a junction 20 which is connected to a ground terminal 18 of a transformer casing 36, and which thus is connected to the casing 36. The ground terminal 18 is connected to the ground and the both terminals ends of the secondary windings 16, 17 are connected to output terminals 21, 22, across which sign lamps 23 such as neon tubes or argon tubes are connected. An alternating current power or commercial power is input across the input terminals 14, 15 and is boosted by the transformer 11 to be applied across the sign lamps 23 for lighting them.
A protective circuit 10 is provided to detect any ground fault, namely, a contact of the sign lamps 23 or their wirings with the casing 36 or a tower on which the sign lamps 23 are mounted and to interrupt the input a.c. power in such event. Specifically, tertiary windings 25, 26 are provided in the vicinity of the secondary windings 16, 17 and are magnetically coupled therewith , and function as part of the protective circuit 10. Usually the tertiary windings 16, 17 are disposed so as to be interposed between the core on which the secondary windings 16, 17 are disposed and the lowermost layer of the secondary windings 16, 17 with a layer of an insulation material having a high withstand voltage capability on the order of 6000 to 7000 V interposed between the secondary windings 16 and 17 and the tertiary windings 25, 26 to provided an enhanced electrical insulation therebetween while assuring a satisfactory magnetic coupling between the secondary windings 16, 17 and the tertiary windings 25, 26.
At one end, each of the tertiary windings 25, 26 is connected together in phase opposition such that their induced voltages cancel each other while the other end of respective tertiary winding 25, 26 is connected to an input of a rectifying and smoothing circuit 27, an output of which is connected through a Zener diode 28 across a parallel circuit including a resitor 31 and a cpacitor 32. A triac 33 has its gate and cathode connected across the parallel circuit. The triac 33 is connected in series with a relay drive coil 34 across the input terminals 14, 15 and the switch 13 comprises a relay contact which is controlled in accordance with the energization of the relay drive coil 34.
Under a normal condition, voltages induced across the teritary windings 25, 26 are substantially equal in magnitude to each other, but are opposite in phase, whereby an input voltage to the recitifying and smoothing circuit 27 is nearly zero. However, upon a ground fault of the sign lamps 23 or the wiring thereof, one of the secondary windings which is associated with the ground fault will be short-circuited, causing a substantial decrease in the induced voltage in the teritiary winding which is coupled with this secondary winding to allow the full induced voltage in the other teritary winding to be applied to the rectifying and smoothing circuit 27. This voltage is rectified and smoothed, and an increase in the rectified and smoothed output voltage turns Zener diode 28 on, with consequence that the triac 33 is rendered on to energize the relay drive coil 34 to open the switch 13, thus interrupting the supply of the input a.c. power to the transformer 11. The switch 13 comprising the relay contact is thrown to the normally open position NO, whereby the holding current to the relay drive coil 34 flows therethrough.
It will be noted that in the described conventional circuit, the pair of tertiary windings are used and disposed below (or inside) the lowermost layer of the pair of secondary windings with a high withstand voltage insulation. The provision of the tertiary windings requires time and labor, reducing the production efficiency of the neon transformer.
Protection against a secondary ground fault of such a neon transformer is also disclosed in FIG. 3 of U.S. Pat. No. 5,847,909 issued Dec. 8, 1998, where the protective circuit does not employ a tertiary winding, but uses an increased number of parts and results in a complicated arrangement, which renders it difficult to utilize a conventional box for containing a neon transformer.
For a neon transformer, it is mandated by legal regulation that the ground terminal 18 be always connected to the ground for use in views of the safety consideration. However, there is a likelihood that a dealer who undertakes constructing a neon tower which uses neon lamps may forget the work of connecting the ground terminal 18 to the ground. A no-ground connection protective circuit which detects such condition during use to interrupt the supply of the a.c. power is proposed and shown in FIG. 2 of the U.S. Patent cited above. However, this no-ground connectin protective circuit again requires an increased number of parts and results in a complicated arrangement, rendering it difficult to utilize a conventional box for containing a neon transformer.
The ground fault protective circuit shown in FIG. 1 detects any ground fault which occurs on the secondary side of the neon transformer immediately to interrupt the supply of the a.c. power to the transformer, and thus is free from any likelihood of causing a fire. However, it is necessary to repair a location where the ground fault has occurred . It will be understood that finding the location of the ground fault is an awful burden for a neon tower of an increased size, for example. If a ground fault causes a ground current to flow to produce sparks, the location of the ground fault may be discovered in a relatively simple manner by relying on light produced or a smell of ozone generated by sparks. However, with the ground fault protective circuit shown in FIG. 1, when a power switch, not shown, is turned off to interrupt the supply of the a.c. power to the input terminals 14, 15 and thus to interrupt the self-holding current to the relay drive coil 34, and the switch is restored to its normally closed position NC before reclosing the power switch, the ground current again flows through the location of the ground fault, which is immediately detected by the ground fault protective circuit 10 to throw the switch 13 to the normally open position NO, interrupting the supply of the a.c. power for the second time. Accordingly, the reclosing of the power switch fails to produce a ground fault condition in order to discover the location of the ground fault. This takes time to identify the location of the ground fault, preventing a repair from being completed rapidly. The same is true with the ground fault protective circuit disclosed in the cited Patent.
It is an object of the invention to provide a no-ground connection protective circuit for a neon transformer which employs a reduced number of parts and a simple arrangement to be capable of detecting a secondary ground fault to interrupt the supply of the a.c. power.
It is another object of the invention to provide a ground fault protective circuit for an neon transformer which temorarily overrides the function of the ground fault protective circuit to enable the discovery of the location of a ground fault to be facilitated.
It is a further object of the invention to provide a no-ground connection protective circuit which employs a reduced number of parts and a simple arrangement to provide a protection against no ground connection of a ground terminal.
A ground fault protective circuit according to the present invention comprises a ground fault voltage detection circuit which detects a voltage equal to or above a given voltage which occurs between a midpoint on a secondary winding of a neon transformer and a ground terminal and a power supply interruption controller which responds to a detection output from the ground fault voltage detection circuit by interrupting the supply of an a.c. power to the transformer.
The ground fault protective circuit of the invention further comprises a ground fault protection stop switch, which overrides the function of the ground fault protective circuit by means of a ground fault protection stop circuit when operated.
With a no-ground connection protective circuit of the invention, a voltage between a non-active line input terminal and a ground terminal is detected, when it is equal to or above a given value, by a no-ground connection voltage detection circuit, and the supply of the a.c. power to the primary winding is interrupted by a power supply interruption controller in response to the voltage detection output.
According to another aspect of the no-ground connection protective circuit of the invention, the application of the power between an active line input terminal and a non-active line input terminal is detected by a power application detection circuit, which is connected in shunt with a switching element. A no-ground connection voltage detection circuit is connected between the active line input terminal and the ground terminal and detects a potential difference between the active line input terminal and the ground terminal to turn the switching element on to override the power application detection circuit. If the power application detection circuit detects the application of the power, the supply of the a.c. power to the primary winding is interrupted by the current supply interruption controller.